1. Field of the Invention
This invention relates to a method for preparing heat-curable silicone rubber compounds in a well dispersed state and at high throughput rates.
2. Prior Art
Heat-cured silicone rubber is improved in weather resistance, durability, heat resistance, physiological activity and colorability. It is used in a variety of applications including building materials, electronic materials, electric materials, business machine parts, automotive parts, and medical parts.
Known methods for preparing heat-curable silicone rubber are to uniformly mix ingredients in compounders. Such compounders include a large size mixer having a pair of mixing blades in a chamber such as a kneader/mixer, a kneader having a pair of rotors in a chamber whereby the contents are kneaded while applying pressure from above to compress the contents such as an internal mixer or Banbury mixer, and a kneader/extruder capable of continuous kneading such as a twin-screw extruder.
Most customarily, large size kneader/mixers having a pair of mixing blades in a chamber are used. The kneader/mixers have the advantage that a large amount of mass can be produced in a single pass on account of its maximum capacity reaching several thousands of liters, but the drawbacks that a long time is needed until the mixture becomes homogeneous and a fully dispersed state is not readily obtainable.
A satisfactory uniform state is reached within a short period of time if a closed mixing apparatus such as a Banbury mixer as illustrated in U.S. Pat. No. 3,951,389 is used. The mixing apparatus of this type, however, has as small a capacity as about several hundreds of liters at maximum. U.S. Pat. No. 4,300,841 discloses a method of compounding a silicone polymer with a filler by mixing the ingredients in a high speed rotation mixer into uniform powder particles and thereafter, mixing the powder in rolls. This method is not applicable when heat treatment is necessary. A continuous production method using a kneader/extruder is disclosed in JP-A 56736/1989. A short working time enables efficient continuous production, but the method results in compounds of variant quality.
It was thus difficult in the prior art to produce heat-curable silicone rubber compounds having improved dispersion and consistent quality within a short time and in an efficient manner.